This invention relates to a novel copper base alloy provided with several excellent properties that render the alloy suitable for use in forming molds used for continuous casting of steel and other metals or alloys.
As is well known, molds formed of tough pitch copper, phosphorus deoxidized copper, or other pure copper base material have mainly been in use for carrying out continuous casting of steel and other metals or alloys, since the techniques of continuous casting have been developed. The principal reason why pure copper base material has been used for forming molds is that the material has excellent thermal conductivity which no other materials possess.
However, any material that is used for forming molds of this type is not considered perfect even if it has high thermal conductivity. Such material should additionally have strength that enables the material to exhibit an excellent anti-thermal deformation ability when liquid steel is poured into a mold and hardness which is high enough to increase the wear resistance of the mold.
When the material is low in thermal conductivity, the temperature difference between the surface of a mold and the water cooled surface thereof would be increased and the thermal stress of the material would increase, causing deformation and crack formation to occur in the mold. Tough pitch copper and phosphorus deoxidized copper that have hitherto been in use for forming molds have the tendency to undergo recrystallization and becoming softer at about 300.degree. C. High hardness is a very important property for material used for forming continuous casting molds. The material is required to have a considerably high hardness to avoid deformation of the mold due to thermal stress of the material and to prevent wear and reduce scratches caused on the mold by the sliding movement of the solidified shell during a casting operation. A lack of hardness of the material would cause diffusion of wear powder into the steel to take place during the sliding movement of the solidified shell, thereby causing star cracks to be formed. When the scratches are large in size, breakout might be caused to occur.
Thus in addition to high thermal conductivity, material for continuous casting molds should be capable of avoiding the occurrence of wear and roughened skin on the inner wall surface of a mold and minimizing thermal strain and thermal deformation of the mold. As set forth hereinabove, tough pitch copper and phosphorus deoxidized copper have been in use over a prolonged period from the time the techniques of continuous casting were initially developed. However, these materials have in recent years raised the problems of deformation and crack formation occurring in molds when they are used in the field of high speed casting that has recently been making advances in which the molds are exposed to severe service conditions, since such materials have hitherto been used at the limit of their characteristics. Thus the present tendency in the metal casting industry is to call for material of high hardness at room and elevated temperatures even at the expense of the ability to transfer heat to a certain degree. This has introduced chromium copper known as precipitation hardenable type material and a C alloy known as a Corson alloy into the field of continuous casting as substitutes for tough pitch copper and phosphorus deoxidized copper of the prior art.
The reason why such materials have become popular is that precipitation hardenable type material has very high strength at elevated temperatures although its thermal conductivity is slightly lower than that of non-aging material, so that molds formed of this material very seldom develop deformation which is a determining factor concerned in the service life of the molds.
The chromium copper is capable of resisting deformation that would be caused by thermal stress produced during a continuous casting operation, but this material is also available only at the limit of its characteristics. The Corson alloy has the risk of developing cracks because it is low in strength at elevated temperatures in spite of being low in thermal conductivity and it is also low in elongation percentage. Thus these two materials lack properties that would make them satisfactorily meet the aforesaid conditions under which the continuous casting mold is forced to operate, and there is an increasingly large demand, among those who are engaged in this technical field, for material of high class for use in forming continuous casting molds.
Continuous casting of steel would tend in the future to be performed on a high speed operation basis so that unit production volume can be increased. In view of this tendency, the problem of deformation would arise with regard to chromium copper, and such material as has increased strength at elevated temperatures even if its thermal conductivity is somewhat low would be in demand.
A Be-Cu alloy in which beryllium is added to copper has been known as a precipitation hardenable type alloy that can be used as material of high strength at elevated temperatures. This material is available commercially as high strength, high heat conductive material. In the case of this alloy, an increase in the proportion of beryllium added to the copper markedly increases strength but reduces its heat conductivity. Conversely, a decrease in the proportion of beryllium, say to below 0.6%, prevents precipitation hardening from occurring. Thus in one type of this alloy, nickel is added to a composition including less than 0.5% of beryllium to lower the solubility of beryllium in copper, to cause precipitation hardening to occur even if the proportion of beryllium is less than 0.6%.
A Cu-Ni-Be alloy is high in strength and high in heat conductivity at room temperature and high in toughness at elevated temperatures, but shows a decrease in strength and elongation, particularly in elongation, when used under conditions in which the temperature rises to the range between 350.degree.-400.degree. C. as in continuous casting apparatus. This also applies to chrominum copper, and these materials always have the risk of being low in toughness when used under conditions of high temperature and high stress.
The Cu-Be-Ni alloy tends to show variations in property because a slight difference in heat treatment for effecting solutionizing and aging can cause a great change in its properties and coarsening of crystal grains. To avoid this disadvantage, proposals have been made to stabilize the alloy by adding cobalt. However, since cobalt adversely affects the heat conductivity of the alloy, the material added with cobalt is not suitable for use as material intended to have high heat conductivity.